The preparation of dense, alumina-based ceramic abrasive grain by sol-gel processes is well known in the art. See, for example, U.S. Pat. Nos. 4,518,397, 4,623,364, 4,744,802 and 4,314,827. Such alumina based ceramic materials are useful as abrasive grains in a variety of products including sand paper, grinding wheels, and other shaped bodies such as monolithic and composite wear components. Vitrified grinding wheels produced from sol-gel grain are known. Shaped bodies formed from sol-gel alumina may be made by a number of different manufacturing techniques including extrusion molding, compression molding, die pressing, and injection molding.
The most economically desirable process for producing sol-gel alumina shaped bodies comprises forming the sol-gel alumina material into a shaped body by a one-step molding process such as an extrusion/injection or an extrusion/compression molding process, or alternatively first forming a multiplicity of small shaped bodies and then compacting them into a desired final shape. Once formed, the shaped body must be dried to remove most of the free moisture from the gelled body prior to final firing. The free moisture removal is necessary because the presence of free water during final firing results in large shrinkage and, more importantly, unacceptable cracking of the body. Thus, the shaped body must be dried to a moisture content of less than about 3.0, preferably less than about 1.0% by weight, before final firing, and the drying must not generate cracks in the shaped body.
However, due to the extremely fine pore structure of the gelled and shaped green body, it is very difficult to remove the water without causing cracking of the shaped body. Some techniques which have been tried to remove the water include controlled humidity drying and atmospheric condition drying by incorporating polymers to prevent cracking. While these techniques do remove the water, they unfortunately also often cause severe cracking of the shaped body and/or require extremely long periods of time, making them impractical and uneconomic.
Because of the cracking problem, abrasive bodies are currently generally formed from sol-gel aluminas by a multi-step process of forming a gel, drying it, crushing the dried gel, sintering the gel particles to form an alpha-alumina grit, mixing the grit with the conventional vitrified or an organic bonding material, pressing the mixture into a desired shape, drying the shape, and firing it at an elevated temperature. The multistep process is time consuming, costly, and, furthermore, the effectiveness of the seeded gel abrasive may not be fully utilized in the presence of the glass bond.
The present invention solves these problems by enabling the production of shaped bodies from an alumina sol-gel by a one or two step process while substantially avoiding the generation of cracks in the dried body.